Field of the Invention
The present invention relates to a method for diagnosing a degree of degradation in a catalyst that oxidizes a predetermined gas component such as nitrogen monoxide.
Description of the Background Art
Conventionally, for on-board diagnostics (OBD) of an exhaust gas cleaning system for vehicles, that is, the function of diagnosing whether a catalyst acts normally or not in the system, a large number of patent applications have been filed mainly by auto companies. Most of these patent applications relate to techniques using an exhaust gas temperature sensor, an oxygen sensor, a wide-range oxygen concentration sensor (λ sensor), a NOx sensor, or a PM sensor, and are targeted for three-way catalysts, oxidation catalysts, NOx storage catalysts, NOx selective reduction catalysts, and diesel particulate filters (DPFs) (for example, see Japanese Patent Application Laid-Open Nos. 2001-263048, 2005-240716, 2012-036860, 2014-62541, and 2010-156243, and Japanese Patent No. 2876793).
Among these, for the OBDs targeted for diesel oxidation catalysts (DOCs), a variety of techniques using various existing sensors such as a temperature sensor, an O2 sensor, a λ sensor, a NOx sensor, and a pH sensor individually or in combination are known at present (for example, see Japanese Patent Application Laid-Open Nos. 2001-263048, 2005-240716, 2012-036860, 2014-62541, and 2010-156243). Specifically, diagnosing techniques using, as indicators, the heat of reaction of a hydrocarbon and oxygen on a catalyst, change in temperature of an oxidation catalyst, an oxygen storage capacity (OSC), a NO/NO2 ratio, and an NO2 ratio in NOx, for example are known.
The techniques disclosed by Japanese Patent Application Laid-Open Nos. 2001-263048, 2005-240716, 2012-036860, 2014-62541, and 2010-156243 all relate to indirect evaluation of an oxidation ability of a diesel oxidation catalyst. Here, there are problems with poor diagnosis accuracy. Furthermore, using a plurality of sensors complicates the system, thus causing a problem with increased cost.
For example, disclosed in Japanese Patent Application Laid-Open No. 2001-263048 is a technique involving the use of a relationship that when the ability of converting (oxidizing or combusting) an unburned hydrocarbon in an oxidation catalyst decreases, exothermic energy will also decrease. In outline, a temperature difference ΔT is measured, which occurs at the time of fuel injection in exhaust gas temperature sensors disposed forward and backward (upstream and downstream) of an oxidation catalyst in an exhaust path, and the degree of degradation in the ability of converting (oxidizing or combusting) an unburned hydrocarbon in the oxidation catalyst is diagnosed indirectly from a measurement value.
In this method, however, a cause of error may be excessively large due to changes in the temperature and the flow rate of an exhaust gas when the gas sensors are practically used, and fuel consumption may inevitably degrade because a large amount of fuel injection is required for accelerating heat generation.
Disclosed in Japanese Patent Application Laid-Open No. 2005-240716 is a technique involving the use of a fact that when the ability of converting an unburned hydrocarbon in an oxidation catalyst decreases, the amount of oxygen consumed during the combustion of oxygen changes. In outline, on the basis of a difference Δλ between output values λF and λR of two wide-range oxygen concentration sensors (λ sensors) disposed forward and backward of an oxidation catalyst in an exhaust path, or a difference between output values (electromotive force values) of two oxygen sensors, an amount of oxygen consumed in an oxidation catalyst is measured, and the degree of degradation in the ability of converting an unburned hydrocarbon on the oxidation catalyst is diagnosed indirectly from a change in the measured value.
However, the concentration of oxygen in a diesel exhaust that is an excessive O2 atmosphere is approximately 10% (=100000 ppm), whereas the amount (concentration) of hydrocarbon that is converted (oxidized or combusted) by an oxidation catalyst is normally on the order of several hundreds of ppm, and the amount (concentration) of oxygen consumed when such a trace amount of hydrocarbon is burned is no more than several hundreds of ppm. This means that the diagnosis of the degradation in an oxidation catalyst with an air-fuel ratio sensor or oxygen sensor requires an accurate calculation of Δλ or a difference in electromotive force corresponding to a ppm-order change in the amount of oxygen consumed, but the air-fuel sensors and oxygen sensors originally cannot achieve such accuracy in measurements.
Even when an unburned hydrocarbon with a concentration of several thousands to several tens of thousands of ppm is produced by an intentional fuel injection, in principle, the sensors disposed forward and backward of an oxidation catalyst do not have any difference in output.
Disclosed in Japanese Patent Application Laid-Open No. 2012-036860 is a technique for determining a degree of degradation in an oxidation catalyst that oxidizes NO to NO2, using an NO2 value obtained based on a ratio of NOx values measured upstream and downstream of the oxidation catalyst in an exhaust path. More specifically, the techniques disclosed by Japanese Patent Application Laid-Open No. 2012-036860 are broadly distinguished into two, in one of which the upstream NOx value is obtained based on a predetermined map and the other of which the upstream NOx value is obtained by a NOx sensor. In both of the techniques, the downstream NOx value is measured by the NOx sensor.
However, in both of the techniques, there is a problem with low diagnosis accuracy, since the degree of degradation in the oxidation catalyst is determined based on a ratio of the two NOx values. Particularly in the former technique, since an estimated value is used as a NOx value for the exhaust immediately after a discharge of an engine or factors except for an engine speed and an engine load are not taken into account in setting of such an estimated value, it is conceivable that the accuracy of estimation will degrade remarkably depending on usage condition.
Disclosed in Japanese Patent Application Laid-Open No. 2014-62541 is a technique in which a NO2 ratio (NO2 concentration/NO concentration) is obtained using a multi-gas sensor including both a NOx detection part and a NO2 detection part and a degree of degradation in an oxidation catalyst is determined based on the NO2 ratio.
However, there is a problem that such a multi-gas sensor has a complicated structure due to including a plurality of sensing electrodes and therefore it is expensive.
Disclosed in Japanese Patent Application Laid-Open No. 2010-156243 is a technique for determining a NO2 ratio in NOx, based on a NO2 concentration in the exhaust that is measured by a NOx sensor and on pH of a condensate liquid obtained by condensing the exhaust, and for diagnosing degradation in a catalyst based on the NO2 ratio. However, since this technique requires a condensation apparatus for obtaining the condensate liquid and a pH sensor besides the NOx sensor, the overall system becomes complicated, which causes a problem with high cost.